Dozens of sites participated, including Aol, The Huffington Post and Digg, and more than 100,000 people and sites have pledged to participate on the Causes.com page for the Web Goes Silent campaign. People and companies are also spreading the word by tweeting their intention to go quiet with the hashtag #momentforSandyHook.

High-profile Silicon Valley venture capitalist Ron Conway promoted the online moment of silence, which is part of a larger campaign for federal gun control legislation. Conway is leading the Causes.com campaign along with other big names including Ryan Seacrest, Jack Dorsey, Britney Spears, MC Hammer, Suze Orman and Tyler Florence.

Causes.com is a startup that uses social media to raise awareness and funds for charities and causes. Conway is also an investor in the for-profit company.

Sites that participated in Friday's moment of silence were invited to do so on their own, or it can embed an official badge with a green ribbon on their sites. Sites using the code appeared grayed out in the background with a white box in the foreground that reads "We are observing a National Moment of Silence for the victims of the Sandy Hook tragedy."

JUST WATCHED

Mom: Speak truthfully about Sandy Hook

MUST WATCH

JUST WATCHED

Family's Christmas without Newtown hero

MUST WATCH

Family's Christmas without Newtown hero09:13

Conway joined other tech notables and some celebrities to demand action for stronger gun control in a full-page ad in the Wednesday print edition of the New York Times. The ad was run by Mayors Against Illegal Guns, a lobbying group of more than 700 U.S. mayors. It started the Demand A Plan campaign to reform gun laws after the Aurora, Colorado, shootings in July, and it has seen a surge in new support after the Sandy Hook shootings.

This is not the first time major sites have banded together to go dark for a cause. This year, major tech names staged an immense and successful online protest against the Stop Online Privacy Act.